The instant invention relates to TV sets and more particularly to a circuit detecting the presence or absence of line synchronization pulses in a composite video signal.
Indeed, among the various circuits constituting a TV set, it is useful, in addition to the circuits designed to form the line scanning signals from the line synchronization pulses, to check if those line synchronization pulses are properly received. The output of a line synchronization detection circuit is liable, for example, to be used for stopping the initial frequency hunting of the TV automatic tuning circuits when a transmission is picked up on the antenna, in order to stop the TV loudspeakers in the absence of a transmission for avoiding to generate noise, or for lowering for a short lapse of time the servo control time constant of the phase locked loop (PLL) of the circuit supplying the line scanning signal in order to accelerate the locking of this loop during the detection of a transmission or a change of channel.
Such circuits are already known in the prior art, an example of which is illustrated in FIG. 1.
In the circuit shown in FIG. 1, the video signal liable to incorporate the synchronization pulses is received on a terminal 1 and sent to a separator circuit of the synchronization signal 2. The output 3 of this synchronization separator circuit is sent to an input of the PLL comprising a multiplier or phase comparator 4, a filter 5 associated with a capacitor 6 external to the integrated circuit, a voltage controlled oscillator (VCO) 8 associated with a quartz 9 external to the integrated circuit, and a divider 10 connected to the output 11 of the VCO and the output 12 of which is connected to the other input of the multiplier 4. The line scanning signal is available at the output 12 of divider 10. This divider 10 is useful because the VCO 8 is liable to operate, not at a frequency close to the desired frequency of the synchronization signal, but at a frequency multiple of the latter, due to the fact one chooses for setting the free frequency of the VCO a frequency range wherein low cost quartz are available.
The video signal detection circuit comprises a coincidence detector 14 receiving the output 3 of the sync separator 2 and the output 12 of the servo control loop. This detector 14 supplies a charging current to a capacitor 16, external to the integrated circuit, when its two input signals are in the coincidence phase, the voltage on this capacitor 16 being compared with a reference voltage by a comparator 17 for supplying the video detection signal on an output terminal 18. Moreover, this figure shows a connection between the video detection signal and the filter 5. This connection is often used for modifying the time constant of the filter. Indeed, in the absence of the line synchronization signal, it is desirable that the loop picks up the signal received as fast as possible, and for this purpose one chooses to impart a short time constant to the filter. But, on the contrary, once the loop is locked, it is desirable to increase the time constant of the filter in order to improve the stability and the noise cancellation.
Because of the continuous progresses and the improvements in quality that are expected from TV sets, it appears that this detection device for synchronization signals according to the prior art is becoming inadequate. Indeed, it presents some drawbacks. In particular, the operation of the detection circuit presupposes the operation of the PLL. Owing to the fact that this PLL operates continuously, even in the absence of synchronization signals on the input terminal of the video signal, a not clearly determined frequency scanning occurs during those phases. This can be a nuisance in some cases when one tries for example to incrust a text into a picture by using the line scanning. On the other hand, as regards the manufacturing costs, this analog-type circuit according to the prior art, necessitates a capacitor 16 having a substantially high value for storing the signals supplied by the coincidence detector 14 and getting rid of the noise. It is well known in the field of integrated circuit manufacturing that, for reducing the manufacturing costs of a circuit, it is essential to increase its integration to the maximum and to avoid using external terminals as well as the high value capacitors that cannot be integrated.
An object of the instant invention is to provide for a video signal detection circuit free from all the drawbacks of the circuits according to the prior art, such a circuit being entirely digital, not requiring the use of an external capacitor and the operation of which is independent of the locking of the PLL.